1. Field of the Invention
The present invention relates to an apparatus for detecting a two-dimensional pattern and a method for transforming the pattern into a binary image, for reducing the noise generated at the digitizing of analog signals obtained from an imaging pickup tube into binary signals.
2. Description of the Prior Art
A two-dimensional pattern testing apparatus has been used for detecting defects in an IC pattern formed on an IC manufacturing photomask or reticle for example by an electron beam exposure. In general such two-dimensional pattern testing apparatus decomposes a two-dimensional pattern composed of bright areas and dark areas into plural pixels and detects said pattern as binary signals respectively corresponding to said pixels. The testing of such two-dimensional IC pattern is however hindered by noise in said binary signals caused by dust deposited on the two-dimensional pattern or by fluctuations of certain parameters in the apparatus.
For the purpose of convenience, the noise in the binary signals is classified into two categories. The two-dimensional patterns include edges constituting the boundaries between light and dark area, and the noise in the binary signals corresponding to the pixels in the vicinity of such edges is called edge noise, while noise in the binary signals corresponding to the pixels distant from such edges is called isolated noise.
In order to detect defects, for example in IC patterns, it is necessary in the two-dimensional testing apparatus to smooth the binary signals, thereby erasing the edge noise and isolated noise and thus obtaining more exact binary signals. Such smoothing has to be conducted in such a manner as to avoid erroneous erasing of small patterns that may be mistaken as isolated noise. In practice, however, when it is required to erase both edge noise and isolated noise it has been necessary to sacrifice edge noise smoothing in order to maintain small isolated patterns, or to sacrifice small isolated patterns in order to effectively reduce the edge noise.